This project studied the responses of trigeminal nucleus caudalis neurons to noxious and innocuous mechanical and thermal stimuli in awake behaving rhesus monkeys. Monkeys were trained to detect the termination of innocuous heat stimuli and the onset of noxious heat stimuli. In a second task, the same monkeys detected the onset of a light stimulus while behaviorally nonrelevant thermal stimuli were presented. Nociceptive neurons exhibited thermal thresholds of 45 degrees C or lower and monotonic stimulus-response functions to noxious stimuli presented during both tasks. Responses to thermal stimuli presented during the light task were reduced in magnitude when compared to responses during the thermal task. A second class of neuron exhibited its maximum activity during the execution of task-related behaviors. This activity was independent of stimulus characteristics and could be separated from motor activity. Nociceptive neurons also exhibited task-related activity, but it was of lesser magnitude than the responses to noxious thermal stimuli. The task-related responses of both classes of neurons may be associated with attentional processes involved in organizing goal-directed behaviors.